Exceptions to the Octet Rule and Variable Valence

Exceptions to the octet rule 

– For a time it was believed that all compounds obeyed the Octet rule or the Rule of Eight.

– However, it gradually became apparent that quite a few molecules had non-octet structures.

– Atoms in these molecules could have number of electrons in the valence shell short of the octet or in excess of the octet.

Examples of Exceptions to the octet rule 

– Some important examples are:

(1) Four or six electrons around the central atom  

– A stable molecule of beryllium chloride, BeCl₂, contains an atom with four electrons in its outer shell.

– The compound boron trifluoride, BF₃, has the Lewis structure :

– The boron atom has only six electrons in its outer shell.

– Beryllium chloride and boron trifluoride are referred to as electron-deficient compounds

(2) Seven electrons around the central atom

– There are a number of relatively stable compounds in which the central atom has seven electrons in the valence shell.

– A simple example is chlorine dioxide, ClO₂.

– The chlorine atom in ClO₂ has seven electrons in its outer shell.

– Methyl radical (CH₃) has an odd electron and is very short-lived.

– When two methyl free radicals collide, they form an ethane molecule (C₂H₆) to satisfy the octet of each carbon atom.

– Any species with an unpaired electron is called a free radical.

(3) Ten or more electrons around the central atom

– Non-metallic elements of the third and higher periods can react with electronegative elements to form structures in which the central atom has 10, 12, or even more electrons.

– The typical examples are PCl₅ and SF₆.

– The molecules with more than an octet of electrons are called super octet structures.

– In elements C, N, O, and F the octet rule is strictly obeyed because only four orbitals are available (one 2s and three 2p) for bonding.

– In the elements P and S, however, 3s, 3p, and 3d orbitals of their atoms may be involved in the covalent bonds they form.

– Whenever an atom in a molecule has more than eight electrons in its valence shell, it is said to have an expanded octet.

Variable Valence

– Some elements can display two or more valences in their compounds.

– The transition metals belong to this class of elements.

– The Electronic Structure of some of these metals is given below:

– Most of the transition metals have one or two outer-shell electrons and they form monovalent or bivalent positive ions.

– But because some of the d electrons are close in energy to the outermost electrons, these can also participate in chemical bond formation.

– Thus transition metals can form ions with variable valence.

– For example, copper can form Cu¹⁺ and Cu²⁺ ions and iron can form Fe²⁺ and Fe³⁺ ions.

– The complete electronic configuration of an iron atom is:

– It can form Fe²⁺by losing two 4s electrons,

– When iron loses two 4s electrons and one of the three 3d electrons, if forms Fe³⁺ion

– Copper form Cu¹⁺ and Cu²⁺ ions by losing one 4s electron, and one 4s and 3d electron respectively

– It may be noted that the structures of Fe²⁺, Fe³⁺, Cu¹⁺, Cu²⁺, Cr³⁺, etc., are not isoelectronic with any of the noble gases, and hence the d electrons being unstable are available for bond formation.

– The atoms and ions that have the same number of electrons are said to be Isoelectronic.

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolor edition.